1. Field of the Invention
The present invention relates to a tellurite glass as a glass material for an optical fiber and an optical waveguide, and in particular a broadband optical amplification medium using the tellurite glass which is capable of working even at wavelengths of 1.5 .mu.m to 1.7 .mu.m. The present invention also relates to a broadband optical amplifier and a laser device using the broad band optical amplification medium. Furthermore, the present invention relates to a method of splicing a non-silica-based optical fiber and a silica-based optical fiber reliably with the characteristics of low fiber-loss and low reflection.
2. Description of the Related Art
The technology of wavelength division multiplexing (WDM) has been studied and developed for expanding transmission volume of optical communication systems and functionally improving such systems. The WDM is responsible for combining a plurality of optical signals and transmitting a combined signal through a single optical fiber. In addition, the WDM is reversibly responsible for dividing a combined signal passing through a single optical fiber into a plurality of optical signals for every wavelength. This kind of transmitting technology requires a transit amplification just as is the case with the conventional one according to the distance of transmitting a plurality of optical signals of different wavelengths through a single optical fiber. Thus, the need for an optical amplifier having a broad amplification waveband arises from the demands for increasing the optical signal's wavelength and the transmission volume. The wavelengths of 1.61 .mu.m to 1.66 .mu.m have been considered as appropriate for conserving and monitoring an optical system, so that it is desirable to develop an optical source and an optical amplifier for that system.
In recent years, there has been considerable work devoted to research and development on optical fiber amplifiers that comprise optical fibers as optical amplification materials, such as erbium (Er) doped optical fiber amplifiers (EDFAs), with increasing applications to the field of optical communication system. The EDFA works at a wavelength of 1.5 .mu.m where a loss of silica-based optical fiber decreases to a minimum, and also it is known for its excellent characteristics of high gain of 30 dB or more, low noise, broad gain-bandwidth, no dependence on polarized waves, and high saturation power.
As described above, one of the remarkable facts to be required of applying the above EDFA to the WDM transmission is that the amplification waveband is broad. Up to now, a fluoride EDFA using a fluoride glass as a host of an erbium-doped optical fiber amplifier has been developed as a broad amplification band EDFA.
In U.S. Pat. Nos. 3,836,868, 3,836,871, and 3,883,357, Cooley et al. discloses the possibility of laser oscillation to be caused by tellurite glass containing an rare earth element. In this case, however, Cooley et al. have no idea of forming tellurite glass into an optical fiber because there is no description concerned about the adjustment of refractive index and the thermal stability of tellurite glass to be required for that formation.
In U.S. Pat. No. 5,251,062, Snitzer et al. insists that tellurite glass play an important role in extending the EDFA's amplification band and it should be formed into a fiber which is absolutely essential to induction of an optical amplification. Thus, they disclose the allowable percent ranges of ingredients in tellurite-glass composition in a concretive manner. The tellurite-glass composition includes a rare earth element as an optically active element and can be formed into a fiber. More specifically, the tellurite-glass composition of Snitzer et al. is a ternary composition comprising TeO.sub.2, R.sub.2 O, and QO where R denotes a monovalent metal except Li and Q denotes a divalent metal. The reason why Li is excluded as the monovalent metal is that Li depresses thermal stability of the tellurite-glass composition.
In U.S. Pat. No. 5,251,062, furthermore, Snitzer et al. make a comparative study of fluorescence erbium spectra of silica and tellurite glass and find that the tellurite glass shows a broader erbium spectrum compared with that of the silica glass. They conclude that the ternary tellurite glass composition may allow a broadband amplification of EDFA and an optically active material such as praseodymium or neodymium may be added in that composition for inducing an optical amplification. In this patent document, however, there is no concrete description of the properties of gain, pump wavelength, signal wavelength, and the like which is important evidence to show that the optical amplification was actually down. In other words, U.S. Pat. No. 5,251,062 merely indicate the percent ranges of ingredients of ternary tellurite glass composition that can be used in an optical fiber.
Furthermore, Snitzer et al. show that thermal and optical features of various kinds of tellurite glass except of those described in U.S. Pat. No. 5,251,062 in a technical literature (Wang et al., Optical Materials, vol. 3 pages 187-203, 1994; hereinafter simply referred as "Optical Materials"). In this literature, however, there is also no concrete description of optical amplification and laser oscillation.
In another technical literature (J. S. Wang et al., Optics Letters, vol. 19 pages 1448-1449, 1994; hereinafter simply referred as "Optics Letters") published in right after the literature mentioned above, Snitzer et al. show the laser oscillation for the first time caused by using a single mode optical fiber of neodymium-doped tellurite glass. The single mode fiber comprises a core having the composition of 76.9% TeO.sub.2 --6.0% Na.sub.2 O--15.5% ZnO--1.5% Bi.sub.2 O.sub.3 --0.1% Nd.sub.2 O.sub.3 and a clad having the composition of 75% TeO.sub.2 --5.0% Na.sub.2 O--20.0% ZnO and allows 1,061 nm laser oscillation by 81 nm pumping. In this literature, there is no description of a fiber loss. In Optical Materials, on the other hand, there is a description of which the loss for an optical fiber having a core composition of Nd.sub.2 O.sub.3 --77% TeO.sub.2 --6.0% Na.sub.2 O--15.5% ZnO--1.5% Bi.sub.2 O.sub.3 and a clad composition of 75% TeO.sub.2 --5.0% Na.sub.2 O--20.0% ZnO (it is deemed to be almost the same composition as that of Optics Letters) is 1500 dB/km at a wavelength of 1.55 .mu.m (see FIG. 1 that illustrates a comparison between .sup.4 I.sub.13/2 to .sup.4 I.sub.15/2 Er.sup.3+ emission in tellurite glass and .sup.4 I.sub.13/2 to .sup.4 I.sub.15/2 Er.sup.3+ emission in fluoride glass). The core composition of this optical fiber is different from that of a ternary composition disclosed in U.S. Pat. No. 5,251,062 because the former includes Bi.sub.2 O.sub.3. It is noted that there is no description or teach of thermal stability of Bi.sub.2 O.sub.3 -contained glass composition in the descriptions of Optics Letters, Optical Materials, and U.S. Pat. No. 5,251,062 mentioned above.
However, the fluoride based EDFA has an amplification band of about 30 nm which is not enough to extend an amplification band of optical fiber amplifier for the purpose of extending the band of WDM.
As described above, tellurite glass shows a comparatively broader fluorescence spectral band width, so that there is a possibility to extend the amplification band if the EDFA uses the tellurite glass as its host. In addition, the possibility of producing a ternary system optical fiber using the composition of TeO.sub.2, R.sub.2 O, and QO (wherein R is a univalent metal except Li and Q is a divalent atom) has been realized, so that laser oscillation at a wavelength of 1061 nm by a neodymium-doped single mode optical fiber mainly comprising the above composition has been attained. In contrast, EDFA using tellurite glass is not yet realized. Therefore, we will describe the challenge to realize a tellurite-based EDFA in the following.
First, the difference between the objective EDFA and the neodymium-doped fiber laser (i.e., the difference between 1.5 .mu.m band emission of erbium and 1.06 .mu.m band emission of neodymium in glass) should be described in detail.
An optical transition of the objective EDFA is shown in FIG. 2 where three different energy levels are indicated by Level 1, Level 2, and Level 3, respectively. For attaining an objective induced emission from Level 2 to Level 1, a population inversion between Level 1 and Level 2 is done by pumping from Level 1 to Level 3 and then relaxing from Level 3 to Level 2. This kind of the induced emission can be referred as a three-level system.
In the case of the neodymium, as shown in FIG. 3, a four-level system can be defined that a final level of the induced emission is not a ground level but a first level (Level 1) which is higher than the ground level. Comparing the three-level system with the four-level system, the former is hard to attain the population inversion so that an ending level of the induced emission is in a ground state. Accordingly, the three-level system EDFA requires enhanced optically pumping light intensity, and also the fiber itself should be of having the properties of low-loss and high .DELTA.n. In this case, the high .DELTA.n is for effective optically pumping.
Secondly, we will briefly described that an amplification band cannot be extended even if it is possible to perform an optical amplification when a transmission loss of fiber is large.
Wavelength dependencies of the silica-based EDFA and the tellurite-based EDFA are illustrated in FIG. 4. As shown in the figure, it can be expected that the tellurite-based EDFA will attain a broadband optical amplification broader than that of the silica-based EDFA. Comparing with the silica-based glass and the non-silica-based glass, a transmission loss at a communication wavelength of the latter is substantially larger than that of the former. In the optical fiber amplifier, therefore, the loss leads to a substantial decrease in gain.
As schematically shown in FIG. 5, if the loss is comparatively small, the amplification band of tellurite glass is close to the one shown in FIG. 4. If the loss is comparatively large, on the other hand, the amplification band of tellurite glass is narrowed.
In recent technical investigations on WDM transmission, by the way, it has been made attempts to speed up transmission through one channel for increasing transmission capacity. To solve this problem, it is necessary to optimize the chromatic dispersion characteristics of the Er-doped optical fiber. Up to now, however, no attention have been given to that characteristics.
For the tellurite glass, a wavelength at which a chromatic dispersion value takes zero is in the wavelengths longer than 2 .mu.m. In the case of a high NA (Numerical Aperture) fiber to be used in EDFA, a chromatic dispersion value is generally -100 ps/km/nm or less at 1.55 .mu.m band. Thus, a chromatic dispersion of a short optical fiber of about 10 m in length also takes the large value of -1 ps/nm or less.
For the use of tellurite EDFA in long-distance and high-speed WDM transmission, therefore, it is need to bring the chromatic dispersion close to zero as far as possible. As described above, however, as the material dispersion value of tellurite glass takes the value of zero at wavelengths of 2 .mu.m and over. Therefore, the tellurite-based optical fiber cannot utilize the technique adopted in the silica-based optical fiber that brings the chromatic dispersion value at 1.55 .mu.m band close to zero by optimizing the construction parameters of the fiber.
Furthermore, the tellurite-based optical fiber can be used as a host of praseodymium (Pr) for 1.3 .mu.m band amplification. As described above, however, the tellurite-bade optical fiber has a large chromatic dispersion value as the absolute value. In the case of amplifying a high-speed optical signal by using the tellurite-based optical fiber, a distortion of pulse wavelength can be induced and thus the chromatic dispersion value should be corrected for. If not, the use of tellurite glass in an optical communication system falls into difficulties.
Next, an optical-fiber splicing between a non-silica-based optical fiber and a silica-based optical fiber will be described below.
For using the above non-silica-based topical fiber such as a tellurite optical fiber as an optical amplification or nonlinear optical fiber, there is a necessity to connect to a silica-based optical fiber to form the junction between these fibers with low-loss and low reflection. However, these fibers have their own core refractive indexes which are different from each other. If these fibers are connected together as shown in FIGS. 6 and 7, a residual reflection can be observed so that the junction appropriately adaptable to practical use cannot be implemented. In FIGS. 6 and 7, reference numeral 1 denotes a non-silica-based optical fiber, 2 denotes a silica-based optical fiber, 5 denotes an optical binder, and 6 denotes a binder. In FIG. 6, furthermore, there is no optical binder applied on a boundary surface between the fibers. As shown in FIG. 8, therefore, the existence of residual reflection between the silica-based optical fibers 2a, 2b and the non-silica-based optical fiber 1 degrades the quality of signal because of a ghost (which acts as noise) due to a reflected signal on the connected ends of the fibers. Therefore, the connected portion between those fibers require -60 dB or over as a residual reflection factor for an optical amplifier (see Takei et al. "Optical Amplifier Module", Okidenki Kaihatu, vol. 64, No. 1, pp 63-66, 1997). For example, a zirconium-doped fluoride fiber, an indium-doped fluoride fiber, chalcogenide glass fiber (i.e., glass composition: As--S), and a tellurite glass fiber have their own core's refractive indexes of 1.4 to 1.5, 1.45 to 1.65 and 2.4 and 2.1, respectively, depending on the variations in their glass compositions. If one of those fibers is connected to a silica based optical fiber (core's refractive index is about 1.50 or less), a return loss R can be obtained by the formula (2) described below. In this case, the unit of R is dB and the residual reflective index is expressed in a negative form while the return loss is expressed in a positive form as an absolute value of the residual reflective index. The return loss can be obtained by the equation (1) below. ##EQU1##
where n.sub.NS and n.sub.S are core's refractive indexes of silica and non-silica optical fibers, respectively. The return loss between the silica-based optical fiber and the zirconium-doped fluoride optical fiber, indium-doped optical fluoride fiber, chalcogenide glass fiber (i.e., glass composition: As-S), or tellurite glass fiber is 35 dB or more, 26 dB or more, 13 dB, or 16 dB, respectively. In the case of Zr-based and In-based fluoride optical fiber, the return loss can be increased (while the residual reflection coefficient can be decreased) by bringing their refractive indexes to that of the silica-based optical fiber's core by modifying their glass compositions, respectively. However, the modification of glass composition leads to the formation of practical optical fiber under the constraint that the glass composition should be precisely formulated in the process of forming a fiber in a manner which is consistent with an ideal glass composition for the process of forming a low-loss fiber). A coupling between the silica-based optical fiber and the non-silica-based optical fiber has the following problems. That is, conventional fusion splicing procedures cannot be applied because of the difference in softening temperatures of both fibers (i.e., 1,400.degree. C. for the silica-based optical fiber and less than 500.degree. C. for the non-silica-based one); the conventional optical connector coupling technologies cannot be applied because there is no appropriate coupling method for the non-silica-based optical fiber; and so on. Thus, a general coupling method for coupling the Zr-based or In-based optical fiber to the silica-based optical fiber without depending on its glass composition has been demanded. In addition, a general coupling method for reliably coupling the chalcogenide glass optical fiber or the tellurite optical fiber to the silica-based optical fiber with a low-loss and low-reflection.
One of the conventional coupling technologies for solving such problems, Japanese Patent Application Laying-open No. 6-27343, is illustrated in FIGS. 9 and 10. In this technology, a non-silica-based optical fiber 1 and a silica-based optical fiber 2 are held in housings 7a and 7b, respectively. The fibers 1, 2 are positioned in their respective V-shaped grooves on substrates 8a, 8b and fixed on their respective housings 7a, 7b by means of bonding agents 10a, 10b and fiber-fixing plates 9a, 9b. In addition, there is a dielectric film 18 applied on a coupling end of one of the housings for preventing a reflection to be generated by coupling the fibers together. The coupling between the non-silica-based optical fiber 1 and the silica-based optical fiber 2 are carried out by using an optical bonding agent 5 made of ultraviolet-curing region after adjusting the relative positions of the housings 7a, 7b so as to match their optical axes. At this moment, the coupling end of the housing 7a is perpendicular to the optical axis of the non-silica-based optical fiber and also the coupling end of the housing 7b is perpendicular to the optical axis of the silica-based optical fiber, so that if the reflection of light is occurred at a boundary surface of the coupling the reflected light returns in the reverse direction, resulting in a falloff in the return loss. Accordingly, the conventional technology uses the dielectric film 18 to reduce the reflection from the boundary surface of the coupling. However, the conventional coupling requires a precision adjustment to a refractive index of the optical biding agent 5 and a refractive index and thickness of the dielectric film 18. That is, their refractive indexes must satisfy the following equations (2) and (3) if a core's refractive index of the non-silica-based optical fiber 1 is n.sub.1 and a core's refractive index of the silica-based optical fiber 2 is n.sub.2. A refractive index of the optical binding agent 5 is adjusted to n.sub.1, while a refractive index and a thickness of the dielectric film 18 is adjusted to n.sub.1 and t.sub.f, respectively, so as to satisfy the following equations (2), (3). ##EQU2##
wherein .lambda. is a signal wavelength (i.e., the wavelength to be used).
In the conventional technology, as described above, there is the need for precisely adjusting a refractive index of the optical binder 5 and a refractive index and thickness of the dielectric film 18 for constructing a coupling portion with the properties of low-reflection and low-loss by using the dielectric film 18. It means that the precise adjustments leads to difficulties in implementing a coupling between the fibers favorably with an improvement in yield.
A process of coupling two different optical fibers in accordance with another conventional technology, as shown in FIG. 11, comprises the steps of: holding an optical fiber 9a on a housing 7b and also holding an optical fiber 9a on a housing 7b; positioning these housings 7a, 7b in their right places so that a coupling end of the housing 7a that holds the optical fiber 19a and a coupling end of housing 7b that holds the optical fiber 19b are positioned with a .theta.-degree slant with respect to a direction perpendicular to the optical axes of the optical fibers 19a, 19b; and connecting the housing 7a and the housing 7b together after the positioning of the housings so as to concentrically adjust the optical axes in a straight line. The process is a so-called slant coupling method for realizing the coupling with low-reflection and low fiber-loss. However, this process is only applied to the fibers when their core refractive indexes are almost the same, so that it cannot be applied to the coupling between the non-silica-based optical fiber and the silica-based optical fiber which have different core refractive indexes with respect to each other.